In the previous post, we discussed some of the built in mechanisms that help to prevent mutations. Sometimes, however, mutations occur in spite of these systems. There are a few different ways that mutations are typically classified in an intro biology course and there are often overlaps between these categories. Here is a brief review of some of the ways that you may see mutations classified:

Mutations can be Neutral, Beneficial, or Deleterious

1. Neutral

Neutral mutations do not help or harm the organism. Certain kinds of mutations are more likely to lead to a neutral mutation. For example, silent mutations (described below) are by definition neutral, because they do not result in a change to the phenotype. Another example of a neutral mutation is a mutation that results in a change in phenotype, but the change in phenotype does not help or harm the organism (for example a mutation that results in a change in hair color).

2. Beneficial

While mutations are often dangerous for organisms (and there are lots of systems in place to prevent them from occurring) they can also be beneficial. For example, mutations occasionally arise in a population that are protective against an infectious disease such as malaria.

3. Deleterious or Harmful

Mutations are often harmful, for example a mutation that damages the cell’s DNA checking mechanisms may increase the likelihood of the organism developing cancer

Mutations can be Silent, Missense, Nonsense, or Frameshift

1. SilentSilent mutations are mutations that do not result in a change in phenotype.

This can occur if:

A change in the nucleotide sequence does not result in a change in the corresponding amino acid. For example if a UUU codon is changed to a UUC codon, this would be a silent mutation because both UUU and UUC correspond to the amino acid phenylalanine.

A mutation occurs in an exon (a non-coding section of DNA), and therefore does not affect the amino acid sequence of the resulting protein

A change in nucleotide sequence results in a new amino acid, but one that shares the same properties of the amino acid it is replacing so that the overall protein continues to function normally

2. Missens

A mutation in the DNA causes one amino acid to be swapped out for another.

This results in a change in the primary structure of protein that can be beneficial, neutral, or deleterious.

3. Nonsense

This type of mutation causes a change in nucleotide sequence that results in an early stop codon (UAA, UAG, UGA).

This can lead to a shortened, incomplete polypeptide

Nonsense mutations are usually pretty major mutations and are often deleterious.

4. Frameshift

A frameshift mutation is caused when a nucleotide is added or deleted. Because codons are read in groups of three nucleotides starting from the end of the last codon, these mutations alter every subsequent codon. See below for an example of how a frame shift mutation alters down stream amino acids.

As you can see above, frameshift mutations usually change the resulting DNA in a very significant way and, as a result, are often deleterious.

One important note about frameshift mutations: If nucleotides were added or removed in multiples of three, a mutation would still have occurred but it would not be a frameshift This is because such a mutation would not impact the way that all downstream codons are read. For example:

So far we have talked about mutations in the nucleotide sequence of DNA. It is also possible, however, to have errors in the overall chromosomes themselves. These mutations can be classified as deletion, duplication, inversion, insertion, or translocation mutations. These types of mutations are covered in less detail in most biology courses, so here is a brief overview of what each of those terms mean:

1. DeletionA segment of a chromosome is deleted

2. DuplicationA segment of a chromosome is duplicated and reinserted into the chromosome

3. InversionA segment of a chromosome is flipped upside down and reinserted into the chromosome

4. InsertionA segment of one chromosome is removed and inserted into another chromosome

5. TranslocationSegment from two chromosomes switch. You can also think of this as two insertions between a pair of chromosomes!

We hope this review was helpful--happy studying!

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